The present invention relates to drainage of slurry from a polishing pad employed in chemical mechanical polishing. More particularly, the present invention relates to through-pad drainage of slurry from a chemical mechanical polishing pad.
Chemical mechanical polishing (sometimes referred to as xe2x80x9cCMPxe2x80x9d) typically involves mounting a semiconductor wafer faced down on a holder and rotating the wafer face against a polishing pad mounted on a platen, which in turn is rotating or moving linearly or orbitally. A slurry containing a chemical that chemically interacts with the facing wafer layer and an abrasive that physically removes that layer is flowed between the wafer and the polishing pad or on the pad near the wafer. In integrated circuit (IC) wafer fabrication, this technique is commonly applied to planarize various wafer layers such as dielectric layers, metallization layers, etc.
Slurry flow during chemical mechanical polishing of IC device silicon wafers can play an important role in uniformity of removal of material from the wafer surface. With exposure time to the wafer surface, the slurry""s chemistry becomes neutralized and its abrasive particles are alteredxe2x80x94being either agglomerated or broken apart. The slurry also becomes loaded with by-products, which are either in solution or suspended particles. All of these factors reduce the effectiveness of the slurry with exposure time.
On a rotary or a linear polisher, wafers move over the moving pad surface. Slurry is provided directly to the polishing pad surface from a source disposed above the pad. Therefore, the slurry is exposed to the edge of a wafer first, and the center of the wafer always sees xe2x80x9coldxe2x80x9d slurry. On an orbital polisher with through-the-pad slurry injection, the slurry flow is generally out toward the edges of the wafer, as governed by centripetal forces and slurry pressure distribution. Depending on injection hole distribution, the dwell time (the length of time the slurry spends on the pad""s polishing surface) will vary. The edge of the wafer will see fresh slurry from nearby injection points plus xe2x80x9colderxe2x80x9d slurry injected near the wafer center. It should be noted that all by-products must flow past the wafer edge to exit the wafer-pad interface.
The quality and effectiveness of chemical mechanical planarization is a function of several factors including slurry application rate, distribution of slurry flow across the polishing pad, the dwell time of slurry on the polishing surface, and the slurry drain rate from the polishing surface. In conventional CMP, some of these parameters may be controlled with some degree of certainty. However, others such as the slurry flow across the pad and the slurry drain rate from the pad are not subject to any fine level of control.
FIG. 1A shows some major components of a chemical mechanical polishing (CMP) apparatus such as an Avantgaard 676, commercially available from Integrated Processing Equipment Corporation (IPEC) of Phoenix, Arizona. CMP apparatus 100 includes a wafer carrier 128 that is fitted with an air chamber 126 (shown in phantom lines), which is designed to secure a wafer 124 by vacuum to wafer carrier 128 during wafer loading typically before CMP is to commence. During CMP, however, wafer 124 is bound by xe2x80x9cwear ringsxe2x80x9d (not shown to simplify illustration) within wafer carrier 128 such that a wafer surface that is to be polished contacts a polishing pad 102. During CMP, the polishing pad 102 orbits while the wafer 124 rotates.
A conventional polishing pad 102 for use with an apparatus such as illustrated in FIG. 1A includes a plurality of slurry injection holes 120, and adheres to a flexible pad backing 104 which includes a plurality of pad backing holes 118 aligned with the slurry injection holes 120. A slurry mesh 106, typically in the form of a screen-like structure, is positioned below the pad backing 104. An air bladder 108 capable of inflating or deflating is disposed between a plumbing reservoir 110 and the slurry mesh 106. The air bladder 108 pressurizes to apply the polishing force. A co-axial shaft 112, through which a slurry inlet 114 (shown by phantom lines) is provided to deliver slurry through the plumbing reservoir 110 and the air bladder 108 to the slurry mesh 106, is attached to the bottom of plumbing reservoir 110. Slurry is delivered to the system by an external low pressure pump. In this configuration, a slurry flow path is defined by the slurry entering through slurry inlet 114, spreading out through the slurry mesh 106 below the pad backing 104, entering pad backing holes 118 and exiting through slurry injection holes 120 on the surface of polishing pad 102. Slurry is distributed on the pad surface by centripetal force, the polishing action, and slurry pressure distribution on the pad 102.
A CMP pad used in a slurry injection system is typically provided with grooves in its polishing surface for slurry distribution and improved pad-wafer contact. These grooves are of two types, either or both of which may be present on a conventional pad""s polishing surface. The smaller of the two groove types, sometimes referred to as xe2x80x9cmicrogrooves,xe2x80x9d are typically about 10 mils wide and 10 mils deep. Microgrooves increase the pad roughness and thereby facilitate the polishing process by creating point contacts and providing space for a small amount of slurry at the wafer-pad surface interface during CMP. Larger or xe2x80x9cmacrogroovesxe2x80x9d (also referred to as slurry distribution grooves) increase the amount of slurry that may be applied to the polishing pad surface per unit area, and thereby increase CMP efficiency. Conventional macrogrooves are typically about 50 mils deep by 50 mils wide.
FIG. 1B shows a top view of a conventional polishing pad 102, such as used with the slurry injection CMP apparatus shown in FIG. 1. An example of such a pad is the IC 1000, commercially available from Rodel Inc., Newark, Delaware. Polishing pads may be made of materials including, for example, urethane, polyurethane, felt, polymer and a filler material. Polishing pad 102 includes macrogrooves (slurry distribution grooves) 130, which are shown in an X-Y configuration, and microgrooves 132 which oriented diagonally relative to macrogrooves 130. At various intersections of grooves 130 in the X direction and grooves 130 in the Y direction, slurry injection holes 120 are provided.
FIG. 2A shows some major components of an alternative chemical mechanical polishing apparatus 200 in which slurry is not injected through the pad to the polishing surface, but is instead applied directly to the polishing surface 212 by a conduit 206 positioned above the pad 220. An example of such an apparatus is the Avantgaard 472, commercially available from Integrated Processing Equipment Corporation (IPEC) of Phoenix, Arizona. CMP apparatus 200 includes a wafer carrier 202, which is designed to secure a wafer 210 during CMP. The carrier 202 is connected to a shaft 204 which moves the carrier 202 towards or away from the polishing pad 220 and rotates and translates the carrier 202 and wafer 220 during polishing.
As shown in FIG. 2B, a conventional polishing pad 220 used in this type of CMP system is not typically provided with grooves in its polishing surface for slurry distribution. These pads 220 may have small xe2x80x9cmicrogroovesxe2x80x9d 222, about 10 mils deep and 10 mils wide, to increase the pad roughness and thereby facilitate the polishing process by creating point contacts and providing space for a small amount of slurry at the wafer-pad surface interface during CMP. These pads 220 also do not have a pad backing, but instead are placed on a table or platen 208. During polishing, the platen 208 rotates or orbits with the pad while the wafer 220 in the carrier 202 rotates and translates.
In both of the conventional CMP systems described above, slurry flow across the polishing surface of the pad is largely governed by centripetal force resulting from the rotation of the pad. Used slurry eventually flows off the edge of the pad and is lost. Additionally, control of slurry flow is minimal. Uneven slurry flow can result in uneven polishing and differential material removal rates across the wafer.
Thus, what is needed is a CMP polishing pad which permits greater control over slurry flow across and off the polishing surface and off the pad.
To achieve the foregoing, the present invention provides a chemical mechanical polishing pad which is capable of draining used slurry from the polishing pad surface through the pad. CMP pads according to preferred embodiments of the present invention have slurry drain holes to drain slurry from the pad surface. In various preferred embodiments, the drain holes are combined with drain grooves in the pad surface and/or the pad/pad backing or pad/platen interface to provide a path for used slurry to exit the pad.
Preferred embodiments of the through-pad slurry drainage system of the present invention also improve slurry flow across the polishing surface. Slurry distribution grooves in the pad surface direct the slurry along their paths. These slurry distribution grooves help prevent accumulations of fresh slurry in regions of the pad surface, particularly those immediately adjacent to injection holes, which could result in local increased material removal and uneven polishing.
The present invention provides a CMP pad having a polishing surface and a bottom surface, and drain holes through the polishing surface to the bottom surface which are capable of providing an exit path for slurry to leave the polishing surface. The pad also includes slurry drain grooves in its bottom surface which are aligned with the drain holes. The drain grooves are capable of providing an exit path for slurry to leave the pad.
The invention also provides a CMP polishing pad having a polishing surface and a bottom surface and having substantially evenly distributed slurry injection holes and slurry drainage holes in the pad. These holes are aligned with concentric circular slurry injection grooves and slurry drain grooves in the pad""s surface. The pad""s bottom surface also contains radial slurry drain grooves which intersect the slurry drain holes.
Another aspect of the present invention is an apparatus for chemical mechanical polishing including a chemical mechanical polishing pad having a polishing surface, a bottom surface and slurry drain holes through the pad. The apparatus also includes a pad backing having a top surface in engagement with the bottom surface of the polishing pad. The top surface of the pad backing has one or more slurry drain grooves aligned with the slurry drain holes in the polishing pad.
A further aspect of the present invention is a platen for supporting a chemical mechanical polishing pad which includes a surface for engaging a CMP pad, and drain grooves in the surface for facilitating removal of slurry from the polishing pad during chemical mechanical polishing.
The invention additionally provides a process of planarization a semiconductor wafer. The process involves providing a slurry to a chemical mechanical polishing pad surface, polishing a semiconductor wafer with the CMP pad, and draining used slurry from the polishing pad surface through the pad.